1. Field of the Invention
This invention relates to a blood oxygenator of the outside perfusion type using a hollow fiber membrane.
2. Description of the Prior Art
A number of blood oxygenators using a hollow fiber membrane have already been proposed, for example, in U.S. Pat. Nos. 2,972,349, 3,794,468, 4,239,729 and 4,374,802.
In these blood oxygenators, homogeneous hollow fiber membrane composed of a gas-permeable material such as silicone or microporous hollow fiber membrane composed of a hydrophobic polymeric material such as a polyolefin are used to bring blood into contact with gas through the medium of the hollow fiber membrane and to effect gas exchange therebetween There are two types of blood oxygenators: the inside perfusion type in which blood is passed through the bores of the hollow fibers while gas is passed on the outside of the hollow fiber and the outside perfusion type in which, conversely, gas is passed through the bores of the hollow fibers while blood is passed on the outside of the hollow fibers.
In most of the conventionally known blood oxygenators, a cylindrical housing is simply packed with a large number of semipermeable hollow fiber membrane for use in gas exchange in such a way that the hollow fibers are parallel to the axis of the cylindrical housing. However, blood oxygenators of this construction have a low gas exchange rate per unit area of hollow fiber membrane, whether they are of the inside perfusion type or of the outside perfusion type. As an improved form of the outside perfusion type, U.S. Pat. No. 3,794,468 has proposed a blood oxygenator in which hollow tubular conduits of semipermeable membrane are wound about a hollow, cylindrical core having a large number of pores in the wall and then contained in a housing, and blood is allowed to flow out of the cavity of the core through its pores while gas is passed through the bores of the hollow tubular conduits.
In blood oxygenators of the inside perfusion type in which gas exchange is effected by passing blood through the bores of the hollow fibers while passing gas on the outside of the hollow fibers, channeling of the blood occurs less frequently. However, since the blood flowing through the bores of the hollow fibers moves in a laminar flow, the internal diameter of the hollow fibers needs to be reduced in order to increase the oxygenation rate (i.e., the oxygen transfer rate per unit area of membrane). For this purpose, semipermeable hollow fiber membrane having an internal diameter of the order of 150-300 .mu.m have been developed for use in blood oxygenators.
However, as long as the blood moves in a laminar flow, the oxygenation rate cannot be greatly increased by reducing the internal diameter. Moreover, as the internal diameter becomes smaller, clotting (i.e., blockage of the bore due to the coagulation of blood) may occur more frequently and/or the blood will be more subject to hemolysis due to an increased pressure loss through the oxygenator, thus posing serious problems from a practical point of view. Further, since a blood oxygenator generally uses tens of thousands of hollow fibers of semipermeable membrane made into a bundle or bundles, special consideration must be given so as to distribute the gas uniformly to the external surfaces of each of these numerous hollow fibers. If the gas is not distributed uniformly, the carbon dioxide desorption rate (i.e., the carbon dioxide transfer rate per unit area of membrane) will be reduced. On the other hand, in blood oxygenators of the outside perfusion type in which gas is passed through the bores of the hollow fibers while blood is passed on the outside of the hollow fibers, the gas can be distributed uniformly and the blood can be expected to move not in a laminar flow. However, these oxygenators have the disadvantage of being subject to insufficient oxygenation due to channeling of the blood and/or blood coagulation at the sites of stagnation. Although the blood oxygenator of the aforementioned U.S. Pat. No. 3,794,468 has undergone improvements in this respect, it is still disadvantageous in that the priming volume of blood is excessively large, a considerable pressure loss through the oxygenator is caused, and a complicated procedure is required for its manufacture. Thus, it remains desirable to develop a more improved blood oxygenator.